MRI vs fMRI

MRI and fMRI both are nouns.

Word	Noun	Adjective	Verb	Adverb
MRI	Yes	No	No	No
fMRI	Yes	No	No	No

As nouns, fMRI is a hyponym of MRI; that is, fMRI is a word with a more specific, narrower meaning than MRI:

MRI: the use of nuclear magnetic resonance of protons to produce proton density images
fMRI: a form of magnetic resonance imaging of the brain that registers blood flow to functioning areas of the brain

Other hyponyms of MRI include functional magnetic resonance imaging.

MRI (noun)	fMRI (noun)
the use of nuclear magnetic resonance of protons to produce proton density images	a form of magnetic resonance imaging of the brain that registers blood flow to functioning areas of the brain